Solid state imagers consist of two principal parts; an image-forming part (i.e., a lens) and an image-capture part (i.e., an electronic image sensor device). The image is projected by the lens onto the surface of the electronic image sensor device at which surface the image is divided uniformly into many small picture elements or "pixels". These pixels are quite small and typically range in size from less than ten microns to over 100 microns across. The electronic image sensor itself is typically a silicon chip upon which an array of photodiodes has been fabricated such that each pixel is associated with a single photodiode element.
A subset of electronic sensors are known as "interline" devices. In these devices the area of the photodiode is significantly less than that of the pixel. A lightshield is placed over the pixel area except in the photodiode region to prevent light from entering other light-sensitive device elements such as the transfer gate or the shift register which are within the pixel area but outside the photodiode. Consequently any light that falls on a particular pixel outside the photodetector area can not be captured unless some sort of optical condensing element such as a lenslet is positioned between the light source and the photodetector. The desired characteristics of the lenslet array are 1) the lenslets must be properly aligned with respect to the photodiode array, 2) the lenslets must be properly spaced from the photodiode, or, alternatively, the index of refraction and the radius of curvature must be such that the resulting focal length of the lenslet is approximately equal to the distance between the lenslets and the photodiodes, 3) the lenslets must be optically transparent and remain so in ambient conditions, 4) the lenslets must be as closely spaced as possible to minimize non-captured light, and 5) the lenslets must be as uniform as possible.
Integrated microlens structures and fabrication processes were disclosed by Y. Ishihara et al., "A High Photosensitivity IL-CCD Image Sensor with Monolithic Resin Lens Array," International Electron Devices Meeting, 1983, pp. 497-500, for cylindrical lenslet arrays and Popovic et al., U.S. Pat. No. 4,689,291, for spherical lenslet arrays. Aspects of both of these methods are summarized in FIG. 1. According to FIG. 1A, a thick layer of photosensitive resin is deposited on an organic planarization/spacer layer 14 and then photolithographically patterned into stripe-like (former case) or cylinder-like lenslet precursor structures 12 (latter case). The organic planarization/spacer layer 14 itself is deposited directly on the surface of the solid-state image sensor. It will be understood that the planarization/spacer layer 14 could be deposited on top of other layers which have been deposited on the surface of the solid state sensor. These additional layers could include patterned dye color filter arrays (such as might be found in color solid state imaging devices), light shield layers, or other planarization layers. These cylinder-like lenslet precursor structures 12 are then heated sufficiently to cause them to reflow, thereby forming semi-cylinder or semi-spherical lenslets 16 (see FIG. 1B). There are several problems with this way of forming lenslet arrays. First of all, especially for electronic color imagers, typical photosensitive resins contain components which absorb in the blue region of the visible spectrum. This results in a distortion of the color spectrum or "yellowing" of the scene that is "seen" by the photodetector array through an adjacent array of color filters. Moreover, the color distortion increases with time due to oxidation of the resin. A second difficulty with this method of forming lenslet arrays is that the resolution with which the photosensitive resin can be patterned is limited by the thickness of the resin layer. The thicker the resin layer, the farther apart the lenslets in the array and, consequently, the less the light collection efficiency of the array. On the other hand, the resin layer must be thick enough so that, when reflowed, the sag of the resultant lenslets is sufficient to cause the desired focusing effect. Accordingly, it is not possible to obtain the highest possible collection efficiency with lenslet arrays fabricated in this manner.
Alternative lenslet fabrication techniques have been proposed which avoid some of the difficulties mentioned above. In these techniques the photolithographic patterning and lens-forming functions are separated. The photosensitive resin serves as both the patterning means and the lens-forming means in the aforementioned technique.
FIG. 2 illustrates an alternative scheme disclosed by Y. Hokari in Japanese Kokai Patent Application No. Hei 4[1992]-226073. In the figure, a lenslet-forming layer 18 is made of a transparent inorganic material such as SiO.sub.2 which is deposited on the surface of an organic planarizing spacer layer 14 which is in turn deposited on the surface of the electronic image sensor chip. A thick photosensitive resin layer is then deposited on lenslet-forming layer 18 and patterned to form lenslet precursors 12 (FIG. 2A). The resin precursors are subsequently transformed into semi-spherical shapes 16 by thermal reflowing (FIG. 2B). As shown in FIG. 2C, inorganic transparent lenslets 20 are then created by transferal of the lens-shaped pattern formed by the reflowed resin to the lenslet-forming layer 18 by reactive ion etching. Next, SiO.sub.2 is deposited selectively only on the surfaces of the inorganic transparent lenslets 20 by means of well-known "spin on glass" or SOG techniques to form lenslet covering films 22 which is shown in FIG. 2D. In this fashion, the spacing between the lenslets can be effectively reduced to zero thereby increasing the light capturing efficiency of the lenslets. One difficulty with this method is the formation of thick layers (i.e. 10 microns) of inorganic materials such as SiO.sub.2 especially on organic bases. Deposition techniques such as RF (radio frequency) sputtering require several hours of deposition time to form such thick layers. During such lengthy deposition times the organic base can become hot enough to decompose. Furthermore thick layers of inorganic materials tend to be mechanically unstable and crack or peel, especially when deposited on organic bases. In addition, films of SOG can require treatment at temperatures as high as 400.degree. C. in order to become fully densified. Clearly the high temperatures associated with thick film inorganic layer deposition are incompatible with both the electronic image sensor substrate and the organic layers. Finally, it is extremely difficult to control the reactive ion etch conditions in order to transfer the lens pattern in the organic resin faithfully to the inorganic layer. Exact etch conditions must be found for which the etch rates of the organic and inorganic materials are identical. Deviations from these conditions or non-uniformity of the etch conditions will result in the formation of unacceptable lenslet arrays. Reactive ion etching can also result in unacceptably rough lenslet surface finishes.
Another alternative scheme for lenslet fabrication is described by H. Kawashima et al. in Japanese Kokai Patent Application No. Hei 3[1991]-297167. Referring to FIG. 3, a thick (i.e., several microns) transparent inorganic planarization/spacer layer 24 is deposited on the surface of the electronic image sensor chip. A thick (i.e., 2 to 10 micron) lenslet-forming layer 26 is then deposited on the planarization/spacer layer 24. This lenslet-forming layer is made of a transparent thermoplastic resin such as PMMA (polymethylmethacrylate), PGMA (polyglycidylmethacrylate), PMIPK (polymethylisopropenylketone), etc. Next a first photosensitive resin layer 28 is deposited on the lenslet-forming layer as shown in FIG. 3A. After photolithographically patterning the first photosensitive resin layer, the pattern is transferred to the lenslet-forming layer by means of oxygen plasma etching (FIG. 3B). The inorganic planarization/spacer layer 24 behaves as an etch-stop for the oxygen plasma etch process. The first photosensitive resin mask 30 is removed with a release solution (ethanol, acetone, etc.) leaving lenslet precursor structures 32 in the thermoplastic resin. The transparent microlens array is then created by thermally reflowing the lenslet precursor structures to form semi-spherical structures 34 which are shown in FIG. 3C. The disclosure goes on to describe the application and patterning of a second photosensitive resin etch-mask 36 which is shown in FIG. 3D. A wet etch solvent is used to remove unwanted portions of the transparent inorganic planarization/spacer layer 38 and the residual resist is removed with a solvent such as ethanol as shown in FIG. 3E. Patterning of the planarization/spacer layer is necessary in order to provide access to electrical contacts of individual electronic imagers. Several of these imagers are fabricated on a single silicon wafer which is subsequently diced into separate devices. If the planarization/spacer layer is patterned prior to spin-coating the lenslet-forming layer, nonuniform coating of the lenslet-forming layer results. This in turn causes nonuniformity of the lenslet array. The fabrication sequence described in FIG. 3 is designed to avoid this problem.
As mentioned with respect to the previously cited reference of Hokari, the deposition of thick inorganic layers is usually problematical due to the long deposition times or high processing temperatures or both. The Kawashima reference also suffers from the fact that most organic photosensitive resins are not immune to erosion by oxygen plasma etching. This lack of selectivity implies that the resin mask will erode as the thick thermoplastic resin layer is being etched and will result in poorly formed lenslet precursor structures and consequently poorly formed semi-spherical lenslets. Even if the selectivity of the photosensitive resin were to be sufficient to withstand exposure to the lengthy oxygen plasma etch process, the difficulty of removing the residual resist prior to reflowing the thermoplastic resin and after patterning the inorganic planarization/spacer layer is problematical. Solvents that are used to remove the photoresists (i.e., ethanol, acetone) can also dissolve the organic thermoplastic resin.